hi, guys!
i have a 2009 g37 with 182k on it.
from some time I mentioned an empty coolant tank and added some coolant to MAX level.
Temp level on dash is always normal.
i started to hear the movement of the coolant and air in the system from the cabin, but I read some forums and it’s didn’t appeared as a problem.
In few days smell a coolant and checked under the hood. I found the spots of dried coolant on the both air filter housings as it were made by the belt, when drops fell on it while rolling.
As the highest point for the drops of coolant was the upper coolant hoose, i really found some drops there and the hoose was like little worn. I changed it and added coolant.
in one day tank was empty and the same traces of coolant buy the point of leak was plastic connector from radiator to upper coolant hoose. I changed it and again added coolant to MAX level.
then I found thet when the car is hot the upper hoose it under high pressure and it is so hot! It seemed to me as the problem was in Radiator cap that didn’t return the excess of hot coolant into the tank and it was changed as well. The sound of coolant and air movement from the cabin disappeared.
Since then I made 600 miles (NYC-Boston-NYC) in one day and it didn’t leak.
But another day I again found that traced of dried coolant on the filter housings again. So, there are some leaks again from under the new radiator cap.
as I understand that the problem is in high pressure. I checked the termostat and it is in working condition. Coolant temp is always normal. No overheating.

don’t know what to do...

 

You should pressure test the system. That will help you identify where the leak is coming from.

 

You can get a loaner tool from any of the big automotive parts stores.

 

There is dye you can put in the coolant and use a blacklight. they make the dye for oil/transmission too. Worked for me as it really stands out. It's at any parts store.

 

It could be as simple as the radiator cap. If it doesn't seal properly coolant can simply evaporate.